A layer of cells that lines the blood vessels and lymph vessels of the body is known as the endothelium. Although we are probably unaware of its existence it is very important for our survival. All blood and lymph carrying vessels, heart chambers, and glomeruli or filters of the kidney are included in the endothelium.
There are four major types of animal tissue including connective tissue, muscle tissue, nervous tissue, and epithelium. The endothelial cells are also a type of epithelial cell. Epithelial cells have a good ability of selective secretion, absorption of substances, and transportation of substances between cells.
These cells can also sense the changes to the cellular environment. Due to all properties, epithelial cells are the excellent choice to line blood and lymph vessels. The balance and maintenance of white blood cells, water, nutrients, and other substances are vital for the proper functioning of blood and lymph.
The sensing ability of epithelial cells helps to sense any changes in the blood and lymph balance and instruct the body. The endothelial cells also play a role in infiltration by the kidney, releasing hormones by the glands, and in the movement of immune cells.
It also helps in the dilating of blood vessels in response to hormones such as adrenaline. The cell has a squashed appearance thus known as a squamous cell. The word squamous is derived from the Latin word ‘squama’ means ‘scale’. They are named due to their similarity with the scales of fish or snakes.
Function of Endothelium
Blood is often described as a “river of the body’ that supplies all messages and nutrients between cells and organs. The river has to be tightly regulated and can’t be allowed to get too high or too low. It also has to be decided which message and material are sent along with the blood.
Any cell or organ of the body only function after receiving the proper message through the blood, thus the functioning of the entire body depends upon the blood. Under a microscope or during dissection, the endothelium looks a bit like cellophane plastic wrap. But it performs many important tasks at the molecular level that are essential to maintain our health.
Some important function of endothelium includes:
- Monitoring and controlling the release of electrolytes and water into the blood.
- It responses to hormones and neural signals by relaxing and contracting to control blood pressure.
- Prevents blood clotting inside the vessels due to the presence of anti-clotting factors.
- The cells are selectively permeable that means they allow only white blood cell to pass between the tissues and blood vessels but does not allow bacteria.
- Helps in the growth and repair of blood vessels.
- Respond to immune and inflammatory signals.
The condition in which the endothelium doesn’t do any job properly is called “endothelial dysfunction”, which can cause many diseases in the body. The failure of endothelial cells to release proper anti-clotting factors can cause high blood pressure and the formation of blood clots in the arteries.
Endothelial problems can also cause inflammatory immune responses that damage organs. The dysfunction of endothelium is common in patients with diabetes, cardiovascular disease, high blood pressure, and high cholesterol.
Examples of Endothelium
i. Artery and Vein Lining
Arteries and veins are the major blood vessels of our body. Our heart pumps a huge volume of oxygenated blood through arteries to our organs at high pressure, thus they are called “fast lane”, whereas veins are known as “return lane” because they carry deoxygenated blood from all organs to the heart.
The arteries and veins have some differences from each other. For example, veins consist of thin walls and are located closer to the skin whereas arteries have thick, muscled walls that help in their pumping action and are located deep under the skin to protect them from injury.
However, both arteries and veins have endothelial linings that help in regulating blood chemistry. According to the body’s needs, the endothelial cell instructs the arteries to relax and contract and also control the release of water, electrolytes, and other substances into the blood.
The epithelial lining of the heart is called the endocardium. The word endocardium is derived from the Greek word that means inside and heart. Besides all the important functions similar to the endothelium, the endocardium also plays an important role in protecting the organ.
It is also thought by scientists that the endocardium controls the flow of substances between the blood and the heart and prevents the heart from harmful substances such as bacteria.
iii. Glomeruli of the Kidneys
Another essential function of endothelial cells is to help the kidneys to filter blood. The kidneys have the function to remove waste products from the blood, these cells allow salts, proteins, and water to pass freely from the blood into the urinary collection system.
Thus, our urine output is increased by drinking a lot of water or by consuming salt. The greater amount of water and salt increases the urine collection through the endothelium.
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- The vascular endothelium and human diseases. Int J Biol Sci . 2013 Nov 9;9(10):1057-69.
- Endothelium structure and function in kidney health and disease. Nat Rev Nephrol . 2019 Feb;15(2):87-108.